The objectives of this fJroposal are to tdevelop and pilot an outreach intervention, incorporating a peer-mediated referral system, and aimed at increasing HIV and syphilis testing among female workers in areas outside of the main entertainment district in Tijuana, IVlexico. The K99 phase of the award examined the individual, network and geographic factors associated with syphilis among IDU in this region. The high rates of syphilis among female IDU in particular, and the high prevalence of methamphetamine use highlight the need to increase connections of this marginalized population to the health care system. Previous work has identified high concentrations of IDU and FSW in the main entertainment district;however, venues In other neighborhoods have also been identified. To date, few women from these locations have participated in studies and therefore little is known about their substance use and risk for HIV/STIs. The current proposal aims to uncover barriers to accessing sexual health care in this population using formative research and to evaluate a peer-mediated outreach intervention using a neighborhood-level randomized controlled trial. A standard outreach intervention, incorporating sexual health messages developed with the community, will be compared to an enhanced intervention incorporating a Respondent-Driven Sampling (RDS)-based peer-referral system. Spatial and envirnomental factors (location, distance, built environment) of the mobile outreach intervention will also be evaluated. A geographically representative sample of women from each neighborhood will be prospectively followed in order to determine the ability of the peer-driven method to increase i) proportion of women accessing the intervention, ii) diversity and representation of women accessing the intervention and ill) neighborhood-level changes in HIV/STI knowledge and risk behaviors. This is in line with NIDA's Division of Epidemiology, Service and Prevention Research to promote "integrated approaches to understand and address interactions between individuals and environments that contribute to the continuum of problems related to drug use".